A non-intuitive shadow spike jets upwards and leftwards from the top of the mountain shadow.

The near full moon slides downwards in the west.

Shadow spikes appear when climbers are below the summit.

Imagine the mountain sliced in two at the climber's level. The mass of rock beneath casts a long shadow. Looking along and through it makes it appear as the 'conventional' triangular mountain shadow with its point upwards.

What about the mountain slice above the climber? It too casts a long shadow tube just like the lower mountain. When the eye looks along it, the upper shadow also has a triangular shape - but pointing downwards. If the summit is to the right then there will be a stronger spike in that direction.